Joined: Aug 2017 Posts: 2,748 Threads: 31
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Location: Auckland, NZ
I have been involved with Jowett Javelin cars for 50 years so, following on from the Seven, have given the topic of crank failure some thought!
At 60 mph, a mile a minute, Sevens are doing in excess of 4,000 rpm, or 4,000 revs per mile. So over whole life average say 5,000 revs per mile. 100,000 miles, which most non sport model original 2 brg cranks exceeded in stock engines, is 500 million revs. In tests of steel samples at constant loading a stress less than that giving a life of 10 million cycles usually ensures everlasting life. Presumably stresses seldom seriously exceed the fatigue limit. So 10 million or less of 500 million revs cause failure, 1/50th of use. The question is which? Is it the 50 mph full throttle, 50 mph overrun, 35+mph in 3rd, the 20 mph in top with advanced spark etc?
A small increase in stress hugely reduces life ie 10% may halve, and many have cr raised more than that. Rev related stress goes up with square of revs; so a 5% rev increase raises stress 10%.
(Jowett cars had cracked or broken cranks after Le Mans, 10 million revs including testing? If anyone here is interested I can very briefly recount the saga from and to reasonable reliability of the production Javelins with 5 variants of factory crank, including finally a nitrided one.)
Joined: Feb 2018 Posts: 367 Threads: 14
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Location: Western Brittany (France)
As usual very interesting reflections Bob. Thanks. And Javelin's saga too if others agrree.
Joined: Aug 2017 Posts: 926 Threads: 22
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Location: Near Cambridge, UK
Car type: 1928 tourer (mag type), short chassis Gould Ulster
The late Ray Walker who spent most of his working life repairing and restoring Sevens told me that two bearing cranks usually go at somewhere around 28 or 56 mph. This suggests that there is a resonant frequency, and maintaining the relevant engine revs for too long will encourage failure. Another learned metallurgist friend and Austin Seven enthusiast confirmed that any piece of steel is likely to break after a particular number (usually very high) of bending actions has been applied to it. Combining these theories suggests that all shafts will fail in the end. Modern techniques such as nitriding are proved to extend the life, but I presume there are still limits as to how long a component will last.
Robert Leigh
Joined: Aug 2017 Posts: 3,004 Threads: 168
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Location: Sherwood Forest
Car type: 1938 Talbot Ten Airline
I have always understood that 28mph in top gear was to be avoided at all costs; this advice was passed on to me by my father, a keen motorist and qualified mechanical engineer. I have never queried it, but have often wondered whether that speed related to 4.00x17 tyres and a 5.25 axle ratio, or 3.50x19 and 4.9 ratio!
Joined: Aug 2017 Posts: 1,337 Threads: 34
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Location: Cheshire
Car type: Race Ulster, 1926 Special, 1927 Chummy, 1930 Box
I think, Mike, that within the limits of speedo accuracy its near enough the same
Alan Fairless
Joined: Aug 2017 Posts: 3,391 Threads: 106
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Location: Darkest Bedfordshire
I'm no metallurgist but I have always thought the small fillet radii used on A7 cranks to be an important factor, often aggravated by poor regrinds back in the day. Whatever the stress is in the journal it's concentrated at least 50% higher in the fillet. The myriad bending actions Robert alludes to are no smaller for want of crank support.
Other than that the greatest enemy is 'enthusiasm' - over-advanced ignition adds to stresses massively and yes, high compression ratios; 6 or 6.5 used to be reckoned the highest that would give reliable service.
I was taught that ferrous metals have a fatigue limit - that is to say there is a level of stress below which they will give infinite service. I was also taught there's many a slip 'twixt cup and lip...
Joined: Aug 2017 Posts: 60 Threads: 5
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Location: Christchurch NZ
Kia Ora Bob. Level 2 in sight.
Do you miss Level 4? With the consistently beautiful weather and the complete absence of traffic our riverside street was a stream of walkers and cyclists, mostly family groups.
I have a Citroen GSA with a flat four boxer engine, so am interested in the Jowett experience. Tell all!
The Citroen has roller crankshaft bearings, a crank handle and 3-stud wheels.
So quite like a Seven really.
Joined: Aug 2017 Posts: 1,641 Threads: 93
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Location: Monmouthshire
Well done Ritchie for having a working GSA. There are a good number of GSA engines very much in service, but very few in the original cars. This is a 2CV owner trying to find things to do to delay going out to continue to sort out fuel feed problems. Fuel pump off yesterday, going to have to drop the tank this morning. After taking the dog for a walk, checking the fluid levels in the Ruby, ringing up family members in England and Italy to see how they are...probably run out of excuses by lunchtime.
Joined: Aug 2017 Posts: 984 Threads: 6
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Location: Scottish Borders
I have covered many 10s of thousands of miles in Austin Sevens since 1964. In that time I have only had one crank failure. It was in My Nippy when it was running in. It happened at about 30mph in top on a light throttle in Highbridge, Somerset. It broke across the rear web, nowhere near a journal.
Maybe it's because I always drive with 'enthusiasm'.
It's the only time I've had to be recovered in a Seven. My wife towed me home in the Saab 99 - an amazingly robust car.
Jim
Joined: Aug 2017 Posts: 919 Threads: 18
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Location: North Yorkshire
Don't start me on SAABS, great cars!
